Although gravity or grid snap are useful features, you may find it difficult to place objects exactly where you want them if they are active. (The green LEDs on the buttons indicate when each snap is being applied.) You can enable or disable either snap while you are adding objects by pressing Ctrl+S (grid snap) or Ctrl+G (gravity snap). That way, you can make some points on a line conform to the grid, and turn off smapping to make the rest of the line irregular (or vice versa).